The impact resistance, being the ability to absorb the energy of a high-speed blow without breaking, is a property of great technological importance. The "Encyclopedia of Polymer Science and Technology," Volume13 (1970), Interscience Publishers, pp.193-195, discloses that polyvinylaromatic compounds, such as polystyrene, have improved toughness -- or so-called "impact resistance" -- when a small amount of unvulcanized elastomeric polymer is incorporated therewith. A phase containing the elastomeric polymer is dispersed in a continuous phase of the polyvinyl-aromatic compound. This dispersed phase is, in effect, frozen as uniformly or substantially uniformly dispersed particles in the continuous phase and is responsible for the improved toughness.
The word "polymerization" is used herein to include copolymerization. The words "vinyl-aromatic monomer" are used herein to include aromatic compounds of which the vinyl group is attached directly to the aromatic nucleus. The words "polyvinyl-aromatic compound" are used herein to mean the vinyl polymerization product of the vinyl-aromatic monomer, i.e., polystyrene from styrene.
The elastomeric polymer present in the impact-resistant polyvinyl-aromatic compound is cross-linked and grafted with polyvinyl-aromatic compound to a greater or lesser extent. An impact-resistant polyvinyl-aromatic compound is usually judged by its Izod impact strength, softening point and melt index. The Izod impact strength gives an impression of the ability to absorb the energy of a high-speed blow. The melt index is a rapid control tool for evaluating constancy of the polymer flow behavior in the manufacture of the polymer is long as major changes are not made in the process. The molecular structure of an impact-resistant polyvinyl-aromatic compound is usually judged by its gel content, the content of elastomeric polymer -- the so-called rubber content -- and the gel/rubber ratio derived therefrom, the swell index and the intrinsic viscosity. The gel content is the percentage by weight of the insoluble constituent of the polyvinyl-aromatic compound, after extraction with toluene, calculated on total polyvinyl-aromatic compound. The gel content indicates the degree of grafting of the elastomeric polymer with vinyl-aromatic monomer and the degree of cross-linking of the elastomeric polymer and is also a measure of the amount of polyvinyl-aromatic compound occluded in the dispersed particles. The rubber content is the rubber content of the starting solution. The gel/rubber ratio influences the Izod impact strength: An increase in gel/rubber ratio is usually accompanied by a higher Izod impact strength at constant content of elastomeric polymer. The swell index is the ratio of the weight of the gel swollen in toluene to the weight of dry, toluene-free, gel. The swell index is an approximate measure of the degree of cross-linking of the dispersed elastomeric polymer. Toughened polystyrene of good quality usually has a swell index between 9 and 16. The intrinsic viscosity is determined at 25.degree. C on the toluene extract obtained from the measurement of the gel content. The logarithm of the intrinsic viscosity is a linear function of the logarithm of the average molecular weight of the continuous phase.
British Pat. Specification No. 1,175,262 discloses a continuous process for the manufacture of an impact-resistant polyvinyl-aromatic compound by the bulk polymerization of a vinyl-aromatic monomer in admixture with an elastomeric polymer and in the optional presence of a minor amount of hydrocarbon diluent by polymerization of the vinyl-aromatic monomer in a plurality of polymerization zones interconnected in series, said zones comprising a pre-polymerization zone (as herein defined), an agitated intermediate zone in which the reaction mixture is maintained substantially homogeneous and heat of polymerization is removed therefrom by the evaporation of vinyl-aromatic monomer and of hydrocarbon diluent (when present), and a final polymerization zone in which polymerization is completed to a desired degree of conversion under substantially adiabatic, plug-flow conditions.
The polymerization is started in the pre-polymerization zone with a homogeneous solution of the elastomeric polymer in the vinyl-aromatic monomer. Almost as soon as polymerization starts, a polystyrene phase separates from the solution, giving rise to a discontinuous phase consisting of droplets of polyvinyl-aromatic compound dissolved in vinyl-aromatic monomer, and a continuous phase consisting of elastomeric polymer dissolved in vinyl-aromatic monomer. As the polymerization proceeds, the volume fraction of the polyvinyl-aromatic compound-in-vinyl-aromatic monomer phase increases relative to that of the elastomeric polymer in vinyl-aromatic monomer. At about equal volume fractions, phase inversion occurs, when agitating is sufficiently vigorous to overcome the influence of high viscosity of the system tending to impede inversion. After phase inversion, the solution of elastomeric polymer vinyl-aromatic monomer is the discontinuous phase suspended in a solution of polyvinyl-aromatic compound in vinyl-aromatic monomer. The pre-polymerization zone is defined as the zone in which this phase inversion is performed.
An essential feature of the above-mentioned agitated intermediate zone is the operation under substantially fully "back-mixed" conditions. In order to achieve back-mixed conditions, i.e., efficient intermixing of the reaction mass, the intermediate reactor is provided with an agitator which is of such a size and shape in relation thereto that the agitation of the reaction mass induced by the evaporation of monomer (or monomer and added diluent, when a diluent is used in the process) therefrom is augmented to the extent required to avoid local overheating of the reaction mass. In this way the problem of temperature control of the reaction mass has been met.
An essential feature of the final polymerization zone is the operation under substantially adiabatic, plug-flow conditions, the words "plug-flow" indicating laminar flow (in the absence of intermixing) and no channelling in the reaction mass. Channelling would lead to wide variations in the characteristics of the ultimate polymer product.
U.S. Pat. No. 3,658,946 issued Apr. 25, 1972 to K. Bronstert et al describes a similar process but incorporates two reactors in the pre-polymerization stage in order to control the size of the gel particles. The two pre-polymerization reactors are operated isothermally, the total conversion over the two reactors being held below 50% while the conversion in the first reactor is held below 16%. Here, as in the aforementioned process, the intermediate stage is composed of one reactor.